Delete a variable using a transition

I'm working on an ECN workflow using a parallel approval process. I'd like to set it up so that once the "voting" process is done the person who gives final approval can approve or reject the ECN. What I'd like to happen is if the document is rejected then it deletes the "yes vote". Can I do this with a transition by creating an action to set the variable and leaving the value blank (or just having a space in the value line)? If that wouldn't work how else could I get this to work?

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